In the manufacturing process for integrated circuit devices, the lithography process is frequently used for reproducing circuits and other structures on a semiconductor substrate. As the first step in a lithography process, a photoresist layer must first be coated onto a semiconductor substrate such that an image may be projected and developed on the substrate. The photoresist material is most likely a liquid that must be coated in a very thin layer on top of the semiconductor substrate. In a conventional process for applying a photoresist coating material to a semiconductor substrate, a photoresist coating machine is used. The coating machine generally consists of a sealed chamber constructed by an upper compartment and a lower compartment and a circular shaped, rotating platform having a diameter that is slightly less than the diameter of a semiconductor substrate. The rotating platform is sometimes called a vacuum chuck since vacuum is applied to the platform for holding the semiconductor substrate securely during a spin coating process. The rotating platform is positioned in the coating machine in such a way that a semiconductor substrate may be placed on top in a horizontal plane. During a coating process, the bottom or the uncoated surface of a semiconductor substrate is placed onto the rotating platform. A suitable vacuum pressure is then applied to supply a suction force to the bottom surface of the substrate such that it stays securely on the vacuum chuck even at high rotational speed. The rotating motion of the vacuum chuck is achieved by an axle which is connected to and extended downwardly from the vacuum chuck such that it may be powered by an electric motor to achieve the rotational motion of the chuck.
In a typical photoresist coating process, a desirable amount of a liquid photoresist material is first applied to a top, upwardly-facing surface of the semiconductor substrate by a liquid dispenser that is mounted on a track system while the substrate is being rotated at a low speed on the vacuum chuck. The photoresist liquid spread radially outward from the center of the semiconductor substrate where it is applied towards the edge of the semiconductor substrate until the entire top surface of the substrate is covered with a thin layer. Excess photoresist liquid spun off the rotating wafer during the photoresist application process. The rotational speed of the vacuum chuck and the amount of the photoresist liquid applied at the center of the semiconductor substrate can be suitably determined and adjusted prior to and during an application process such that a predetermined, desirable thickness of the photoresist is obtained. The rotational speed of the vacuum chuck is normally increased to a higher speed at the end of the application process to ensure that the entire surface of the substrate is evenly coated with the photoresist material.
A conventional apparatus for coating photoresist on a semiconductor substrate is shown in FIGS. 1A and 1B. The apparatus 10 shown in FIG. 1A consists of an upper compartment 12 and a lower compartment 14. The compartments are generally of a cylindrical shape such that when put together, a round cylindrical shaped cavity is provided inside the two compartments. A top ring, or roof member 18 is provided in the upper compartment 12 by compression fit. A rotating platform 20, or a vacuum chuck, is positioned at the center of the cavity 22 for supporting a semiconductor substrate 26 on a top surface 24 of the vacuum chuck 20. The vacuum chuck can be rotated by an axle 32 which is connected to an electric motor (not shown) for providing rotational motion. The lower compartment 14 is provided with a spent photoresist drain pipe 34 and an exhaust pipe 36. The spent photoresist drain pipe 34 is used to drain away photoresist liquid that spun off the substrate during a coating operation. The exhaust pipe 36 is used to exhaust the air flow generated by the rotating vacuum chuck to prevent a pressure build up in the cavity 22 of the chamber. It should be noted that, in the conventional photoresist coater, the upper compartment 12 and the lower compartment 14 are assembled together in a fixed position such that the height of the cavity 22 between the upper compartment 12 and the lower compartment 14 is fixed and cannot be adjusted. It should also be noted that the top ring 18 is assembled to the upper compartment 12 in a fixed manner such that its relative position to the semiconductor substrate 26 cannot be adjusted. FIG. 1B shows a perspective view of the top ring 18, the upper compartment 12 and the lower compartment 14 in a disassembled view.
In the operation of the conventional coater shown in FIG. 1A, the vacuum chuck 20 with a semiconductor substrate 26 positioned on top is first elevated to a position near the top ring 18. A liquid dispenser (not shown) then approaches the center of the substrate 26 and applies a predetermined amount of the liquid photoresist material to the center of the substrate 26. The vacuum chuck 20 then starts spinning to spread out the photoresist material to evenly cover the top surface of the substrate 26. Extra photoresist material is thrown off the substrate surface and drained away by the drain pipe 34. An air flow generated between the rotating vacuum chuck 20 and the top ring 18 carries contaminating particles in the chamber and the sprayed photoresist powder into the exhaust pipe 36.
When a lithographic process is carried out on a photoresist layer, any foreign particles or defects in the pattern formed on the photoresist layer act as extra etch mask and are reproduced on the substrate surface. Certain types of these extra resist pattern have been identified as originating from the photoresist coating process, i.e., photoresist powder or gel that bounces back onto the substrate surface during the high speed spinning step of the coating process. The extra resist pattern on the substrate surface causes serious defects in the substrate and thus, greatly reduces the yield of the chip fabrication process.
Modern coater designers have noticed the contamination problem by the photoresist powder or gel and a catch-cup assembly (the lower compartment shown in FIG. 1A) has been installed on the coater in an attempt to eliminate the problem. The lower compartment 14 of the coater consists of a photoresist drain pipe 34 and an exhaust pipe 36 wherein the drain pipe is supposed to collect extra photoresist liquid thrown off the substrate, while the exhaust pipe is to collect photoresist powder sprayed into the cavity of the coating apparatus. However, due to the existence of an air flow turbulence in the cavity, the powder sprayed into the cavity fall back onto the substrate, especially when the powder is mixed with vaporized thinner in the air flow. The sprayed powder in the cavity when fell back onto the substrate surface forms a "donut" shaped extra photoresist pattern on the surface of the substrate. In a donut-shaped pattern, the photoresist layer is thicker at the edges of the substrate than at the center thereof Such formation causes quality and reliability problems since the thickness of coating on the substrate surface varies from the target thickness that is supposed to be formed. Such variations from the target thickness of the photoresist layer across the surface of a semiconductor substrate greatly affect critical dimensions during subsequent processing steps for the substrate. For instance, variations in the photoresist layer from the target thickness may greatly affect the linewidth control of a polysilicon layer. This is caused by the thickness variations of the photoresist layer and thus different linewidths are generated during the photolithography steps. The control of the uniformity of the photoresist coating layer is therefore critical for achieving high yield in a semiconductor fabrication process.
It is therefore an object of the present invention to provide a method for controlling a liquid coating process without the drawbacks and shortcomings of the conventional liquid coating methods.
It is another object of the present invention to provide a method for controlling a liquid coating process on a semiconductor substrate that can be utilized in an existing commercial coating apparatus.
It is a further object of the present invention to provide a method for controlling a liquid coating process by adjusting the cavity height inside a liquid coating chamber.
It is another further object of the present invention to provide a method for controlling a coating process in a photoresist coating apparatus by increasing the distance between a substrate to be coated and the interior wall of an upper compartment such that photoresist particles thrown off the substrate are not bounced back from the wall onto the substrate.
It is still another object of the present invention to provide a method for controlling a photoresist coating process in a coating apparatus by providing an upper compartment and a lower compartment which are capable of forming a cavity therein having adjustable height.
It is yet another object of the present invention to provide a method for controlling a photoresist coating process by utilizing an adjustable top ring in the upper compartment of the apparatus such that the distance between the substrate to be coated and the top ring can be adjusted.
It is yet another further object of the present invention to provide a photoresist coating apparatus consisting of an upper compartment and a lower compartment which can be assembled together and adjusted to produce an adjustable height inside the two compartments.
It is still another further object of the present invention to provide a photoresist coating apparatus that is equipped with an adjustable top ring installed in an upper compartment of the apparatus such that an air flow occurring on top of the substrate can be adjusted.